(LOS ANGELES, CA) -- About 15 minutes into class, the instructor shrieks: ``Are you still wearing clothes? Ladies, take off those shirts!''

Jeffrey Costa pumps his hips with gusto and watches the class bump and grind in the mirror. The ladies hesitate and steal furtive glances at their classmates. One T-shirt drops to the floor. A few others follow.

The wooden floor reverberates to the booming bass line of Destiny's Child's ``Independent Woman'' and Costa shouts encouragement to the group of about 20, punctuated by sharp sighs reminiscent of Michael Jackson in mid-moonwalk.

The routine is a weekly ritual for 31-year-old Costa, who cooks up a different program each week for the class in ``Cardio Striptease,'' billed by fitness temple Crunch as a way to shed both pounds and inhibitions.

Strip aerobics, which debuted in mid-June, marks the New York-based chain's attempt to attract beautiful people and wannabes bored by run-of-the mill aerobics. Other innovations aimed at the same trendsetters have included ``Cycle Karaoke,'' ''Circus Sports'' and ``Gospel Moves.''

The frenzy of innovation reflects an increasingly competitive market. The number of U.S. health clubs rose 6 percent last year to a record high of 16,983, according to an industry group.

``Health clubs are now more than ever making a huge effort to accommodate all different kinds of people, from the elderly with bad knees to the young with the short attention spans,'' said Margo Faiman, a spokeswoman for the International Health, Racquet and Sportsclub Association.

Los Angeles was the launch pad for the aerobics craze of the early 1980s, which was promoted by Hollywood celebrities like Richard Simmons and Jane Fonda, and the city remains an important laboratory for innovation.

``News spreads like wildfire, especially in L.A.,'' model and actress Angelica Bridges says after a recent strip workout. ``If something's hot, hip and happening, then everybody wants to do it, and I think that's what's going to happen to this class.''

EVERYONE'S A WALLFLOWER

``I want you to fall in love with yourself,'' Costa tells his class. Several students shoot sexy gazes and seductive smiles at their reflections. Others maintain looks of fierce concentration, particularly during the raunchiest bits.

Stripping off gym clothes in front of a group of strangers is not everybody's idea of a good workout. Even Bridges, who romped through the television series ``Baywatch'' in swimwear, admits to being a bit apprehensive before her first class.

``I hope there are a lot of shy people who come to this class because that's the point,'' Costa says. ``It's to do things that maybe someone told you were bad or wrong or dirty and say 'You know what, this is human nature.'''

The discomfort level rises visibly during a recent class when a good part of the gym assembled to watch through the studio's glass walls. Some have suggested passing around a hat so bystanders can tip for the show, Costa said.

Most eyes are riveted on his teaching assistant Blayne, who chose to withhold her last name.

Her high-heels clack along the wooden floor as she whirls through the room, pausing to toss her blond ponytail through the air and show some skin. She likes to wear stretchy things that she can ``pull down and play with,'' Blayne says.

First-timer Miri Pardo was a little intimidated by the whole affair. She described herself as ``very shy'' and said that, for her, ``Cardio Striptease'' is as much about boosting self-esteem as it is about losing weight.

``As we were walking in I saw all the plastic and all the make-up and I thought 'uh-oh, am I in the right place?,''' the 36-year-old said.

YOU CAN KEEP YOUR CLOTHES ON

Though the name might suggest otherwise, ``Cardio Striptease'' won't end with everyone bouncing around naked. Most choose to stop at their sports bras and exercise shorts. No one should feel obliged to take their clothes off or even wear anything particularly revealing, Blayne says.

Sometimes it's the quiet ones who turn out to be the wildest.

Blayne says she's often surprised that folks who appear a bit conservative and inhibited at first sight pull out all the stops once the music starts.

And some women who didn't dress up at all for class later confided they felt so inspired that they went home, put on some high-heeled shoes and a bra and danced in front of the mirror.

But if someone should get carried away and start tearing their clothes off in time to the music, Blayne says they'd have her blessing -- though California law enforcement might have something to say about it.

``I think that if someone really got into it and started feeling a little exhibitionistic, then more power to them,'' Blayne says.

Bridges, a red-haired Missouri native who has graced a dozen magazine covers, including an upcoming ``Playboy,'' says she was relieved to find that the class is as much about exercise as it as about exhibition.

``I thought they would be using poles and straddling chairs and doing all this crazy stuff, but it's actually the best workout I've ever had,'' Bridges says.

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